1. Field of the Invention
The invention relates to electrically conducting solid plastics based on polymers which are themselves electrically insulating, and wherein the plastics contain particulate, electrically conducting substances.
2. Discussion of the Background
Ordinarily, the term "plastic material" is associated with the property of high resistance to passage of electric current, i.e. insulation. Conductivities of such materials are in the range G=10.sup.-10 to 10.sup.-18 (ohm-cm).sup.-1. There has been substantial industrial interest in the insulating properties of plastics. However, occasions arise when it is desirable for polymers to be electrically conducting. Polymers which have, for example, regularly alternating double and single bonds as the principal feature of their bonding systems can achieve sigma-values, i.e., units of conductance in .OMEGA.-1 in the metal or semiconductor range, when electron acceptors or electron donors are incorporated in them. See Weddigen, G., Physik in unserer Zeit, 14, 4:98; (1983) and "Kirk-Othmer", 3rd Ed. John Wiley, Vol. 18 pp. 755-93 (1982). Such polymers include, for example, polyacetylene, polypyrrole, and polysulfur nitride. Addition of "conducting fillers" can result in increases in the conductivity of polymers which are inherently insulators, such that technically useful conductivities are achieved. Candidates for such fillers include carbon black, lead, and silver. According to Weddigen (loc. cit.) one can reproducibly achieve only a narrow conductivity range, i.e., between 10.sup.-4 and 10.sup.1 (ohm-cm).sup.-1 with the filler method. The content of conducting filler is generally 10 to 40 wt. %. At relatively low filler concentrations (about 5 wt. %) the conducting particles do not yet statistically form conduction paths within the insulator, so that they do not result in overall conductivity. When the filler concentration is increased, there occurs an abrupt incidence of statistically frequent contacts of conducting filler particles, and consequently an abrupt increase in conductivity to a level close to that of the filler material itself.
At high field strengths, the lines of current flow first run along the paths formed by the filler particles which are in contact with each other. At low field strengths, filler particles which are close but not touching do not contribute to the current through the solid plastic; while at high field strengths there is dielectric breakdown. Such a conductor no longer obeys Ohm's law.
Another disadvantage of the "filler method" is that when the filler is incorporated, the conductor filler particles are not uniformly distributed, due to their density being different from that of the insulating polymer matrix. They are more concentrated in the lower regions. In this regard, "Kirk-Othmer" (loc. cit., p. 767) states: "Doped polymers exhibit a host of additional difficulties associated with the disorder and gross inhomogeneity of the dopants. Thus, achievement of the goal of making synthetic metals from conducting polymers faces hurdles that were unanticipated as little as a decade ago." Homogeneity of distribution of the conductivity carriers thus appears to be an essential prerequisite for industrial use of polymeric conductors. The cited article by Weddigen makes it clear that future prospects are not good for the use of plastic to which electrically conducting filler have been added.